Many packaged microelectronic devices have a substrate, a microelectronic die attached to the substrate, and a protective covering or encapsulant encasing the die. The protective covering is generally a plastic or epoxy compound that can be molded to form a casing over the die. The microelectronic die can be a memory device, a microprocessor, or another type of microelectronic assembly having integrated circuitry. Several types of packaged devices also include bond pads on the substrate that are coupled to the integrated circuitry of the die. The bond pads may alternatively be coupled to pins or other types of terminals that are exposed on the exterior of the microelectronic device for connecting the die to buses, circuits, and/or other microelectronic assemblies.
A significant limiting process when manufacturing packaged microelectronic devices is encapsulating the die with the protective covering. The dies are sensitive components that should be protected from physical contact and potentially harmful environmental conditions to avoid damaging the die. The protective casing encapsulating the die, therefore, should seal the die from the external environment and shield the die from electrical and mechanical shocks. Thus, the protective casing should not have any voids that may allow contaminants or other harmful agents to contact and potentially damage the die.
One conventional technique for encapsulating dies is known as transfer molding, and involves placing the die in a cavity of a mold and then injecting a thermoset material into the cavity. FIG. 1A illustrates a representative transfer molding tool 10 that simultaneously encases a plurality of microelectronic dies 60. More specifically, the molding tool 10 can include an upper plate 30 removably positioned on a lower plate 20 to define a plurality of substrate chambers 14. A plurality of channels 16 connect the substrate chambers 14 to an upright pellet cylinder 17. A cylindrical pellet 40 formed from an epoxy mold compound is positioned in the cylinder 17, and a plunger 15 moves upwardly within the cylinder 17 to transfer heat and exert pressure against the pellet 40. The heat and pressure from the plunger 15 liquefy the mold compound of the pellet 40. The liquefied mold compound flows through the channels 16 (as indicated by arrows F) and into the substrate chambers 14 to surround the microelectronic dies 60. As the liquefied mold compound flows into the substrate chambers 114, it drives out the air within the molding tool through small vents 18. This is known as the transfer stage of the process. During a subsequent compaction stage, pressure is exerted on the encapsulant within the substrate chambers 14 to collapse microvoids that may be present within the encapsulant. The mold compound in the substrate chambers 14 is then cooled and hardened to form a protective casing around each microelectronic die 60.
Another conventional technique for encapsulating dies is compression molding, illustrated schematically in FIG. 1B. During a compression molding process, the die 60 is placed on a lower plate 20, the encapsulant 40 is placed on the die 60, and an upper plate 30 is brought downwardly toward the lower plate 20. The upper plate 30 and the lower plate 20 together form a substrate chamber 14. As the upper plate 30 contacts the encapsulant 40, it forces the encapsulant 40 around the die 60. The amount of encapsulant 40 placed on the die 60 is typically slightly more than is necessary to fully encapsulate the die 60, so as to ensure complete encapsulation. Accordingly, the excess encapsulant 40 escapes from the substrate chamber 14 through escape channels 19.
As part of a continuing effort to streamline the process for manufacturing microelectronic devices, many processes which formerly were performed on singulated microelectronic dies are now being performed on unsingulated dies, including entire wafers of such dies. These processes are typically referred to as wafer-level processing techniques. In some cases, wafer-level processing has been used to at least partially encapsulate the dies at the wafer level. However, existing encapsulation techniques have been difficult to implement at the wafer level. For example, the wafers typically include solder balls, wirebonds, or other conductive structures for connecting the dies to other devices. When a wafer containing such conductive structures is placed in a transfer mold, the rapid flow of encapsulant over the surface of the wafer can sweep away or otherwise disturb the position, shape and/or orientation of these conductive structures. The compression molding process can also suffer from drawbacks. For example, when an encapsulant is disposed on a wafer having solder balls, wirebonds, or other small conductive structures, small pockets of air can be trapped between the encapsulant and the small conductive structures. However, because the compression mold is not completely sealed (so as to allow excess mold compound to escape from the mold cavity via the escape channels 19), the encapsulant in the mold cavity cannot be compacted to collapse these voids. Therefore, for at least the foregoing reasons, it is desirable to improve the process for forming protective casings over microelectronic devices.